1. Field of the Invention
The present invention relates to a color cathode ray tube inner shield and a method for fabricating the same, and more particularly, to a color cathode ray tube inner shield and a method for fabricating the same, which can prevent aberration of electron beams caused by external magnetic field.
2. Discussion of the Related Art
In general, as shown in FIG. 1, the color cathode ray tube is provided with a panel 1 having an inside surface coated with a fluorescent film, and a funnel 2 having an inside surface coated with conductive graphite, both of which are bonded with a fusion glass in a furnace at about 450.degree. C. The funnel 2 has a neck part 3 in which electron guns 7 are mounted for emitting electron beams 9 and deflection yokes 8 mounted on an outside circumference thereof. A shadow mask 4, which is an electrode for selecting a color, is mounted inside of the panel 1 supported by a frame 5. There is an inner shield 6, which is a body for shielding a magnetic field, mounted inside of the shadow mask 4 for preventing aberration of the electron beams 9 due to the terrestrial magnetic field.
The operation of the aforementioned cathode ray tube will be explained.
Upon reception of a video signal, thermal electrons are emitted from cathodes in the electron gun 7 and travel towards the panel 1 while being accelerated and converged by acceleration and focusing electrodes. In this instance, the electron beams 9 are regulated of their paths by the deflection yokes 8 to be directed on a spot of the panel 1. The regulated electron beams are selected of a color while passing through a slot in the shadow mask 4 and collide onto the fluorescent film on the inside surface of the panel 1, to generate a fluorescent light, thereby reproducing the video signal. In the meantime, the electron beams 9 emitted from the electron gun 7 are influenced by the terrestrial field to be deviated from their normal paths. That is, the electron beams 9 passed through the shadow mask 4 are deviated from their normal paths and collide on a color other than intended to degrade an image due to lack of a counter measure for a component of the terrestrial field entering into the color cathode ray tube in a horizontal direction, i.e., in an axial direction of the color cathode ray tube. In order to solve this problem, the inner shield 6, which is a magnetic field shielding body of a feeble magnetism, is provided rear of the shadow mask 4. That is, as shown in FIGS. 2 and 3, an external magnetic field infiltering into the cathode ray tube is induced to flow along the inner field 6, preventing infiltering of the external magnetic field into an inner side of the color cathode ray tube.
However, a complete shielding of the axial component of the terrestrial field of the color cathode ray tube by means of the inner shield has been difficult because of its limit in its magnetic properties. Namely, the inner shield is formed of pure iron, for example, of aluminum killed steel(AK steel) having a carbon content reduced therefrom, by subjecting to continuous casting, hot rolling, cold rolling, and decarburization annealing, to obtain a thin sheet, which is then subjected to drawing or forming to an intended form. As the inner shield is formed of aluminum killed steel having a carbon content reduced therefrom, of the magnetic properties, the inner shield has a permeability, which determines an intensity of a magnetic flux flowing inside of a body, below 3000 and a coercive force below 1.5 Oe. Because a degree of magnetic shielding is dependent on the permeability, the inner shield with 3000 permeability has a limitation in shielding the external magnetic field. Particularly, the external magnetic field infiltering into the cathode ray tube concentrates on corners of the cathode ray tube and, therefrom, inflows into the cathode ray tube, giving an influence to the electron beams to deviate from the normal path, leading to a mislanding of the electron beams, that resulting in degradation of a finesse of an image on the cathode ray tube. Moreover, the severe mechanical deformation at the corners of the inner shield occurred at the thin sheet drawing or forming significantly degrades magnetic properties of the deformed portions, with a non-uniformity in overall magnetic properties of the inner shield, causing the aberration of the electron beams more severe and degrading the finesse more.